The invention relates to a method for regulating the evaporation rate of oxidizable substances in reactive vacuum deposition by the metered addition of oxygen to a vacuum deposition chamber during deposition.
The quality, in particular the optical properties, of deposited oxide layers depends to a very great extent in reactive evaporation on the homogeneity of the layer material within the deposited layer. This applies particularly to deposited multiple layer systems, to which the so-called interference layer systems belong. If it is not possible to deposit layer by layer under the same conditions, i.e. to apply each layer with the same constant composition and at approximately the same deposition rate, then non-homogeneities arise, which adversely affect the dependency of the absorption and transmission behaviour on the light wavelength and in particular largely make it impossible to reproduce the method. The absorption and transmission behaviour over specific wavelength regions of light can be shown diagrammatically, and one talks of absorption and transmission curves.
A basic requirement for uniformity within the individual layers is the uniform oxidation of the layer material by the oxygen introduced into the vacuum chamber. Factors that interfere in the density or pressure of the oxygen in the vacuum deposition chamber are the suction capacities of the vacuum pumps connected to the vacuum chamber, the desorption of gases, changes in the amount of oxygen added per unit time, as well as the constant consumption of oxygen by the oxidation of the evaporant. On account of the continuous addition of oxygen and the continuous evacuation by the vacuum pumps, an equilibrium state is established, which of course may be subject to variations over the course of time. A fluctuating oxygen partial pressure leads, in the case of a constant evaporation rate, i.e. in the liberation of uniform amounts of evaporant per unit time, to a varying oxidation behaviour of the evaporant, with the result that the afore-described variations in the composition of the layer material occur.
Those skilled in the art have therefore always attempted to maintain the feed rate of oxygen to the vacuum chamber as constant as possible. This objective can however be only very unsatisfactorily achieved and at considerable expense and effort, on account of the regulation behaviour of the metering valves required for this purpose. On the other hand, those skilled in the art also attempted to maintain the evaporation rate of the evaporation source as constant as possible by regulating the electrical power. To this end, a whole number of so-called rate measuring devices were developed. However, the oxidation behaviour necessarily varied as a function of the amount of oxygen added per unit time.